Such filter mats and filter elements equipped with filter mats are used for filtering liquids, for example, in the form of hydraulic oil or solutions of any sort, for example, in the form of true suspensions, dispersions, emulsions, or colloidal solutions. The filter elements and filter mats have proved to be universally useful on account of their compact structure and their large filter surface area. The known filter elements, which are readily available through commercial channels, have a zigzag folded or pleated filter mat composed of multiple layers of different filter materials. The filter mat is laid around a solid support tube mounted in the interior of the filter element and is provided with passages. The multi-layered structure of that filter mat may have the following laminate structure, for example, from the one side to the other side:                1. metal wire mesh or plastic woven fabric or plastic screen with reticular structure,        2. polyester nonwoven fabric,        3. glass fiber mat or melt blown nonwoven fabric,        4. glass fiber mat or melt blown nonwoven fabric, paper nonwoven fabric, or polyester nonwoven fabric,        5. steel and polyester mixed woven fabric, and        6. metal wire mesh or plastic woven fabric or plastic screen with reticular structure.        
In the course of an efficient production sequence, such a multi-layered mat web is passed through a cutting unit in the longitudinal direction of the web. In this cutting unit, the mat web is cut to size at its edges before it is passed to a folding machine, where the zigzag form or the pleat is formed. Then in the next phase of the production sequence, the cut web is severed into web sections, from which then the filter mat is made in the form of a tubular body.
In order to ensure that systems that use such filter elements and that often exhibit a high quality operate reliably, it is of crucial importance that the tubular filter mat that is manufactured as the end product of the aforementioned process steps be an absolutely clean component. To put it more succinctly, it must be ensured that the end product is totally free of impurities, like dirt particles, fiber particles, and the like. With respect to the construction of the multi-layered mat web made of glass fiber fabrics, nonwoven fabrics, woven fabrics, or screen structures, these requirements cannot be satisfied during the production process without more effort. Hence, the handling and temporary storage of the mat webs between the working steps result in fiber migration and fiber release, in particular also during the production steps of longitudinal cutting, folding, and cross cutting of the mat web at its edges. This situation poses a problem not only for the cleanliness of the component, but also renders the handling of the mat web and, hence, the automation of the production process more difficult. Automation is also rendered more difficult due to the tendency of the support layers in the mat web to end out.
DE 42 02 769 A1 discloses a filter, in particular a filter that is intended for use in gas filters, made of a zigzag folded and meltable web material with folds that are closed at the ends. The gas filter can be inserted into a housing such that the rims are sealed for separation of a downstream cleanroom. The object is to make it possible to seal the rim at the folded end edges as well as to form a closed frame element as simply and inexpensively as possible. To achieve this goal, the known solution provides that the end edges of the folds are melted and compressed so as to form an enlarged contact surface, thus forming a common, closed frame element as a composite of the individual contact surfaces. Then, sealing strips can be adhesively cemented or welded on this common composite of end edge surface areas in a simple way. The solution according to DE 42 02 769 A1 provides that the web material be welded on its long side. EP 0 522 692 A2 discloses a method of the aforementioned type for constructing a filter mat with a multi-layered web, which is welded on its end sides. With respect to the requirements for fabricating a filter end product of high component cleanliness, the known solutions leave much to be desired.